1. Field of the invention
This invention provides a phase conjugation device and, in particular, a device to generate an optical wave whose phase is the conjugate of an incident wave, in a non-linear medium. In particular, the invented device aims to improve the spatial quality of laser beams by phase conjugation, for example by Brillouin phase conjugation in a high-pressure gas cell. The optical configuration of the invention ensures very high stability of the conjugate wave reflectivity and very effectively limits unwanted effects such as breakdown of the medium and beam autodefocussing.
2. Discussion of Background
In the conventional configuration, represented in FIG. 1, the conjugate wave is produced by focussing the incident wave in a high-pressure gas (CH.sub.4, Xe, N.sub.2, etc.) cell 1. Under these conditions, wave Ep generates a diffraction grating due to the propagation of an acoustic wave generated by the electrostrictive effect at the focal position. The phase of wave Es back-scattered by the diffraction grating is then the conjugate of the incident wave Ep. In an oscillator-amplifier laser structure, phase conjugation dynamically compensates for phase distortion caused by the laser amplifier medium thermal lens. In practice, the quality and stability of the conjugate wave are limited by several phenomena which oppose the Brillouin effect:
defocussing due to residual absorption of the beam in the cell, PA0 the Raman effect, PA0 the autofocussing effect due to the electrostrictive displacement of material around the focal point.
In addition, because the mechanism relies on the light noise diffused by the acoustic phonons in the medium, it is preferable to use a controlled optical signal to initiate the Brillouin back-diffusion phenomenon. This invention aims to satisfy this last requirement and, simultaneously, to reduce the energy densities at the focal point. Consequently, unwanted non-linear effects are considerably reduced, significantly improving the long-term stability of the reflectivity and the precision of the conjugate wave.